It is often desirable to measure the distance from the observer to a target for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Stadiametric rangefinding, or the stadia method, is a technique for measuring distances with a telescopic instrument. The stadia method is based upon the principle that in similar triangles homologous sides are proportional. This means that for a right triangle with a given angle, the ratio of adjacent side length to opposite side length is constant. By using a reticle with marks of a known angular spacing, the principle of similar triangles can be used to find either the distance to objects of known size or the size of objects at a known distance. In either case, the known parameter is used, in conjunction with the angular measurement, to derive the length of the other side.
Since a radian is defined as the angle formed when the length of a circular arc equals the radius of the circle, a milliradian (sometimes called a mil), is the angle formed when the length of a circular arc equals 1/1000 of the radius of the circle. An object 5 meters high, for example, will cover 1 mrad at 5000 meters, or 5 mrad at 1000 meters, or 25 mrad at 200 meters. Since the radian expresses a ratio, it is independent of the units used; an object 6 feet high covering 1 mrad will be 6000 feet distant.
A reticle is a shape superimposed on an image that is used for precise alignment of a device, most notably that of a telescopic sight. The minimum reticle consists of simple crossed lines, or crosshairs, that meet at the optical center of the device. Most commonly associated with telescopic sights for aiming firearms, crosshairs are also common in optical instruments used for astronomy and surveying.
Telescopic sights for firearms, most commonly referred to as scopes, are the devices most often associated with crosshairs. A number of patents have been granted for rangefinding reticles for scopes. Various reticle approaches also exist in the practiced prior art.
While the traditional thin crossing lines are the original and still the most familiar crosshair shape, they are really best suited for precision aiming at high contrast targets because the thin lines are easily lost in complex backgrounds, such as those encountered while hunting. Thicker bars are much easier to discern against a complex background, but lack the precision of thin lines. The most popular types of crosshairs in modern scopes are variants on duplex crosshairs, with bars that are thick on the perimeter and thin out in the middle. The thick bars allow the eye to quickly locate the center of the reticle, and the thin lines in the center allow for precision aiming. The thin lines in a duplex reticle may also be designed to be used as a measure. Called a 30/30 reticle, the thin lines on such a reticle span 30 inches at 100 yards when the scope's power is at 4×. This enables an experienced shooter to deduce (as opposed to guess or estimate) the range within an acceptable error limit.
It is desirable for the aiming point of riflescopes to be at the center of the circular field of view, because this provides a psychological confirmation of the aiming point, as well as providing a rough aiming point in rushed circumstances when discerning the cross hair aiming point is not possible. Moreover, while vertical holdovers tolerate some deviation from the center aiming point, lateral displacements of the aiming point would create a needless conflict with the user's natural expectation that the center of the circle will coincide with the center of aim.
Two examples of scopes utilizing a rangefinding reticle are found in U.S. Pat. Nos. 4,403,421 (Shepherd) and 4,584,776 (Shepherd). These patents disclose a telescopic sight including primary and secondary reticles separately disposed within separate image planes formed at respective opposite ends of an inverting tube. The secondary reticle bears engraved indicia for determining target range and for compensating for bullet drop.
The Shepherd patents feature engraved indicia of military figures for use in connection with military warfare. Each figure is visually associated with the respective bullet-drop compensation aiming point disposed vertically with respect to an engraved dot located centrally in the upper portion of the reticle image. Each of the figures is shaped to indicate the size of an object at a specific range. Furthermore, the figures represent specific distances. Even the head portion of the figures is sized to indicate the range of the object in the event that only the head of the object can be seen through the scope.
The Shepherd patents alternatively feature a reticle illustrated with engraved indicia for use by game hunter. These engraved indicia include a plurality of superimposed circles meeting in a single point on each of the circumferences of the circles. Each of the circles is visually associated with different range distances.
However, there are a number of problems with prior art patents and existing practiced prior art. These problems include obscuring of the object being ranged by the reticle if the reticle is centered. A centered reticle requires the user to place an object that he or she desires to be unobstructed outside the center of the device's field of view. However, in peripheral portions of the field of view, optical performance is degraded.
One of the major problems is that existing rangefinding reticles are employed in scopes attached to firearms. However, using telescopic sights attached to firearms to determine the range of unidentified objects is generally considered to be unsafe; a firearm should never be pointed at an object the shooter does not intend to shoot. Although binoculars and spotting scopes/monoculars (portable telescopes optimized for the observation of terrestrial objects) can be used to identify distant objects safely, they often omit reticles because they are not typically used to aim a firearm. An example of such a spotting scope/monocular is U.S. Design Patent D603,436 (Hamilton), hereby incorporated by reference in its entirety.
All of the above reticles and rangefinding scopes have significant disadvantages in terms of safety and visual clarity, at least for certain applications and needs.
It is therefore an object of this invention to provide a reticle for stadiametric rangefinding that enables spotting scopes and binoculars to measure the distance to an observed object without obscuring the object when it is centrally located in the field of view.